In high-frequency communication systems typified by mobile phones, high-frequency filters have been used to remove unnecessary signals other than signals in the frequency band used for communications. Acoustic wave resonators such as surface acoustic wave (SAW) resonators have been used for the high-frequency filters. In the SAW resonator, a metal grating electrode is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate.
The grating electrode excites a Shear Horizontal (SH) wave, a Rayleigh wave, or a boundary acoustic wave, which is a type of surface acoustic wave. Reflectors are located at both sides in the propagation direction of the acoustic wave excited by the grating electrode to confine the acoustic wave in the vicinity of the grating electrode. Ladder-type filters and multimode filters can be made with acoustic wave resonators. There have been known acoustic wave resonators in which the width in the direction perpendicular to the propagation direction of the acoustic wave is weighted in the grating electrode as disclosed in, for example, Japanese Patent Application Publication Nos. 9-270667 and 2008-78883 (hereinafter, referred to as Patent Documents 1 and 2, respectively).
In acoustic wave resonators having a grating electrode, lateral-mode spurious, which is unnecessary response, is generated. The lateral-mode spurious is generated when the acoustic waves each having a component in the direction perpendicular to the propagation direction of the acoustic wave amplify each other at a certain wavelength. In Patent Documents 1 and 2, since the overlap width changes in the propagation direction of the acoustic wave, the frequency at which the lateral-mode spurious is generated changes in the propagation direction. Accordingly, the frequencies at which the acoustic waves in the lateral mode amplify each other are averaged, and thereby, the lateral-mode spurious is reduced. However, this does not mean that the generation of the acoustic wave in the lateral mode is inhibited. Therefore, the acoustic wave in the lateral mode leaks to the outside of the grating electrode, causing loss.